Consolidation of oil reservoirs is known, but sand production from a weak or badly consolidated oil reservoir is still a serious problem that causes damage to the reservoir itself, wells, pumps etc. It may also be very costly to remove sand from the oil phase at a later stage in the oil recovery process. Water proofing technology used to reduce water production from special zones into oil field wells in order to obtain enhanced oil recovery is also an important issue for oil field operations.
U.S. Pat. No. 5,143,155 mentions use of urease manufactured from bacteria in situ, to reduce porosity and permeability of oil reservoirs, by precipitating CaCO3 from an aqueous solution containing calcium chloride and urea. The object of this US patent is to reduce porosity/permeability of geological subterranean formations to increase oil production from an oil reservoir and to reduce the flow of contaminations from previous operation present in the aqueous phase.
It is taught in SPE publications 50621 (Harris, R. E. and McKay, I. D. New application for enzymes in oil and gas production. The 1998 SPE European Petroleum Conference, The Hague, The Netherlands 20-22 Oct. 1998) that urease may be used to decompose urea to consolidate sand when precipitating CaCO3. It is also mentioned in this publication that calcium phosphate can be produced in a reaction between an enzymatically decomposed phosphate and calcium chloride. The publication concludes that such materials have a potential for sealing water conveying layers and for applications related to sand stabilizing.
The Norwegian patent 326444 describes chemical consolidation of sandy oil formations and water tightening of underground constructions using calcium phosphate or calcium carbonate, and urease from a plant.
From U.S. Pat. Nos. 5,143,155 and No. 6,401,819 B1 and the SPE publication 50621 (Harris, R. E. and McKay, I. D. New application for enzymes in oil and gas production. The 1998 SPE European Petroleum Conference, The Hague, The Netherlands 20-22 Oct. 1998) it is known that CaCO3 precipitation in oil reservoirs through an enzymatic process using urease, a Ca2+ source and urea can improve the formation stability and reduce sand production. It is also known that calcium, urea and urease can form CaCO3 in order to change the porosity and permeability in porous media. See eg. Nemati, M., Greene, E. A. og Voordouw. G. Permeability profile modification using bacterially formed calcium carbonate: comparison with enzymic option. Process Biochemistry 40 (2005) 925-933, and Nemati, M. and Voordouw. Modification of permeability profile, using calcium carbonate produced enzymatically in situ. Enzyme and Microbial Technology 33 (2003) 635-642.
The technological process of performing the well and water tightening treatments, in order to reach the objects stated for oil field- and underground water tightening treatments in large scale is, however, not explained in the above patents and publications in such details that the processes can be done in an economic industrial process. The major problem is the supply of urease in sufficient quantities and in a stable form.
Freeze dried or lyophilized urease is a commercial product, one can buy it from for instance Merck or Sigma. However, these ureases are produced in such a way that they do not function for consolidation, and they are very expensive (about NOK 260.000, for 1 kg). It has not been possible for the applicant to get hold of the methods the manufacturers are using, but it is likely to assume that certain stabilizers have been added to the aqueous solution before lyophilization, and that these stabilizers are the reasons why the urease does not work for consolidation.